Time to ramble! If the title interests you, click below the break for my thoughts on that most powerful, rarest of moments, and when and why it strikes.
An epiphany, as any decent dictionary will tell you, is a sudden flash of insight, when you see something in a new way, or gain some deeper understanding. The word comes from christian literature, where it refers to Jesus appearing to the apostles after his death. That's a pretty good ur-example, and it also highlights the first of the three kinds of epiphany: the shocking or unexpected, and perspective-altering, event.
In the world of literature, the biggest example of this kind of epiphany that I've personally experienced was the end of chapter seven of Lord Foul's Bane (aka "if you've ever read the book, you know exactly what I'm talking about"). It's a shocking, appalling moment which completely, irreversibly alters the way you perceive the title character... which is precisely what it's meant to be. For me, though, the epiphany wasn't just being surprised by where the book went; that hardly constitutes "deeper understanding" by itself. What made it epiphinic to me was that it altered the way I read and understood character archetypes; Thomas Covenant wasn't the first anti-hero I'd ever encountered in fiction, but he was the first one who, for me, broke past "unusually grumpy good guy" and broke apart my understanding of what a hero--what a character with whom the reader is supposed to empathize--was, and what he could and couldn't be. When I finished that chapter, I was angry--my blood was pounding, and I put the book down and didn't pick it back up until the next day--but in that moment, my relationship with protagonists changed forever.
The second kind of epiphany is the kind that only reveals itself based on how ready you are to understand what's written; some epiphanies can't be rushed, and can't really be experienced until one is mentally ready, or sufficiently mature (emotionally or otherwise). Tolkien has given me plenty of these over the years; the inevitable result of first reading the books in elementary school, and revisiting them many times thereafter. Perhaps the biggest, though, was my appreciation for the climax at Mount Doom. From the first time I read it, of course, I grasped the events: Frodo claims the ring, Gollum attacks him, they wrestle, and Gollum and the ring fall into the fires together. I think my first response to that was "Frodo was a wuss, they got lucky with Gollum being there."
But of course, there's a lot more to the ending than that, and it wasn't until a later re-reading that I suddenly realized, with an almost literal flash of insight, that there wasn't anything lucky about it (at least, in a narrative sense). Gollum was there because Sam and Frodo displayed mercy and, in Frodo's case, empathy, and those are transcendent values. Because they showed the power to do what they believed was right, the world was redeemed. After all was said and done, Sauron was right not to fear that his enemies would try to destroy the ring; no man or elf could have succeeded in that quest. And yet the fellowship was redeemed, not through superior might or wit or willpower, but because even in the darkest of moments, Frodo found it within himself to be compassionate, and to exemplify the greatest human values. The moment that I realized that didn't just recontextualize the entire story for me, but changed the way I saw the world around me in a way too fundamental to overstate.
The last kind of epiphany is the one that lurks in words, waiting for repetition to pound their meaning through into your skull. The most dramatic expression of this for me came not from a book, but a poem: Mourning Pablo Neruda, by Robert Bly. When I first read it, my reaction was that it was just another piece of pretentious modern poetry, trying too hard to say too little. Then a choir I sang with performed a musical arrangement of it, which I felt at first was a great fit in that it was also full of pretentious bric-a-brac. But sometime around the twentieth or thirtieth time we were rehearsing, a light bulb went off. I still don't know what changed, but it was as if all my disdain for poem and song alike fell away in an instant. Suddenly, the breaks were the clippings of thoughts cut short by grief. Suddenly, the early circling of the subject was a self-erected bulwark against grief. Suddenly, where only a moment before I'd have angrily denied any comparison, I saw my brother in that poem. I had to excuse myself from rehearsal, and I went to an empty practice room, where I curled up against a wall and cried until I was empty.
Whatever form it takes, the epiphany is one of the most powerful feelings I know of, and anything which evokes that expanding of awareness in me holds a special place in my heart. When I read a story, there are plenty of things I'm looking for, and plenty of things with which I can walk away satisfied. But deep down, whenever I open a book, new or old, what I'm hoping to find is that flash of insight which changes everything. Unpredictable as it is, and personal as it is, it remains the most powerful gift a reader can receive.